The input symbols that are composed by the transmission antenna index, the OAM state and the radiated symbol are discrete symbols, thus they do not satisfy the requirement of Gaussian input for wireless communication systems in general. In this case, the Shannon capacity can not be achieved for wireless communication systems with non-Gaussian input [1]}. On the other hand, because of the additive Gaussian channels in OAM-SM millimeter wave communication systems, the signals received by antenna pairs are continuous signals. Consequently, the discrete input continuous output memoryless channel (DCMC) capacity needs to be derived for analyzing the capacity of OAM-SM millimeter wave communication systems [2]}. Furthermore, the capacity of OAM-SM millimeter wave communication systems is expressed as
\({C_{{\text{OAM-SM}}}}{\text{ = }}\mathop {{\text{max}}}\limits _{\mathcal {P}\left( {{x_p}} \right),\mathcal {P}\left( l \right),\mathcal {P}\left( m \right)} \frac{1}{M}\sum \limits _{m^{\prime } = 1}^M {\sum \limits _{m = 1}^M {\sum \limits _{\ell = 1}^L {\sum \limits _{p = 1}^P {\int _{ - \infty }^{ + \infty } {\left\lbrace  {\begin{array}{*{20}{c}}{\mathcal {P}\left( {{y_{m^{\prime },1}},m,l,{x_p}} \right) \cdot } \\{{{\log }_2}\left( {\frac{{\mathcal {P}\left( {{y_{m^{\prime },1}}\left| {m,l,{x_p}} \right.} \right)}}{{\mathcal {P}\left( {{y_{m^{\prime },1}}} \right)}}} \right)}\end{array}} \right\rbrace } } } } {\text{d}}{y_{m^{\prime },1}}} .\qquad \mathrm {(22)}\) 
